Patent GB-646981-A (GB)
Title: Electrical bridge measuring apparatus
IPC: G01R17/02
Filed: 1944-07-24
Abstract: 646,981. Resistance measurements; strain-sensitive resistances. FOXBORO CO. July 24, 1944, No. 14131. Convention date, July 28, 1943. [Class 37] [Also in Group XXXVIII] In arrangements wherein conditions, e.g. temperature, pressure, flow, electric potential, strain &c. are measured by means of elements the electrical characteristics of which are sensitive to variations in these conditions, the conditionsensitive element X is arranged in a balanced element bridge circuit 10, Fig. 1, and out of balance potentials produced in the bridge output as a result of variations in the condition being measured, are balanced against the output of a balancing bridge 20, the adjustment of an impedance R1 in one of the arms of the balancing bridge, required to produce the output potential to balance the output from the element bridge being a measure of the variation in the condition being measured. A plurality of such element bridges 10, 10a, may be sequentially switched into circuit with the balancing bridge by means of switches S, Sa. The balancing of the two out-ofbalance potentials may be effected automatically in each switch position and the sequential operation of the switches may be effected automatically. In such a case the various measurements are by the balancing arrangements used, rendered substantially independent of the contact resistance of the sequence switches. The adjustment of the variable impedance R1 in the balancing bridge necessary to effect balance of the double (element-balancing) bridge may be recorded on a chart moved automatically to a fresh position for each sequence switch position. The balance indicator 26, Fig. 1, may alternatively be connected across the outputs of the element and balancing bridges, Fig. 2 (not shown), in which case a current balance is effected rather than a potential balance. Fig. 3 shows the application of the invention to measuring the mechanical strain of a body 201 at various positions G, H, ..... Strain gauges 202, 202a, ... are located at these positions and connected in separate element bridges 204, 204a, . . . adapted to be energized and connected in circuit successively with the balancing bridge 242 through switches 220, 220a, .... The element bridges each consist of a fixed resistor 206, two resistors 208, 210 which are simultaneously variable in opposite directions for final balancing of the individual element bridge and fixed resistors 212, 214, which with a switch 238 are used for coarse adjustment. The balancing bridge 242 has located in one arm thereof a temperature - compensating element 262 with leads 264, 266 similar to and subjected to the same temperature conditions as the strain gauges 202 ... , and has resistors 286, 288 for range changing or setting, i.e. adjustment relative to the gauge factor of the strain gauges 202 .... In operation the balancing bridge is individually balanced by adjusting the zeroizing resistance 258 until the balance indicator 282, connected for this purpose directly across the bridge output by a switch 290, reads zero. Switch 290 is then opened and element bridge 204 connected in circuit by closing switch 220. The element bridge is then individually balanced for some predetermined zero position of the strain gauge 202 by the coarse adjustment switch 238, and fine adjustment variable resistors 208, 210. Balance is indicated by the indicator 282 without any adjustment of the balancing bridge 242 so that both bridges 242, 204 are individually balanced. Subsequent variations in the strain gauge resistance 202 cause an output potential from the element bridge which is balanced out by adjusting the slide wire resistor 256 in the balancing bridge. This resistor may thus be graduated to read the strain at the location G of the body 201. The similar element bridges such as 202a may be switched successively in circuit by switches 220a ... when they function similarly to bridge 202. Variable resistors, construction. The variable resistors 208, 210 of the element bridge 204, Fig. 3, are constructed similarly to the strain gauge elements 202, 202a ... and affixed one on each side of a flexible cantilever strip 292, Fig. 4, attached at its free end to a nut 296 threaded upon a rod 300 rotatably supported by collars 302 in a bracket 304. Rotation of the rod 300 thereby deflects the movable end of the cantilever strip 292, thus putting one strain-sensitive resistor under compression and the other under tension to vary their resistances simultaneously but in the opposite sense. Null indicator. Fig. a shows the balance indicator 282, Fig. 3. in detail. The indicator input from lines 234, 278 is applied through a transformer 316 to a three-stage resistancecapacity coupled amplifier 318, the output of which is applied through a transformer 320 to the diagonal points e, g, of a rectifier bridge. The opposite diagonal points d, f, are connected across the secondary ab of the output transformer 228 of oscillator 230 supplying the element and balancing bridges. The rectifier bridge arms are identical so that in the absence of any output from amplifier 318 there is no potential drop across the direct current centre zero meter 330 connected between centre-taps on the transformer secondaries h, m and a, b. The meter 330 deflects in one direction or the other according to the direction in which the slide wire resistor 256 has to be adjusted to the balance position, and with the circuit shown responds substantially only to the " resistive " component of the out-of-balance potential in the double-bridge circuit, i.e. the component in phase with the bridge energizing potential. A capacitor 344 compensates for any phase shift introduced by the amplifier 318. Automatic balancing, recording mechanism. For automatically switching a number of similar element bridges successively in circuit, balancing the double bridge automatically and separately recording the strain in respect of each measuring position the arrangement of Fig. 6 is used. The element bridges are connected in turn to the balancing bridge 350 over lines 225, 223, 240, 234 as in Fig. 3. The self-balancing bridge 350 is energized from winding 384 of the oscillator 356 and the adjusting capacitative arm 372 is varied mechanically by a solenoid motor 362 which also drives a recording pen 380 movable over a chart 382 to record the adjustment. A variable resistance 376 is adjusted according to the gauge factor and zeroizing of the bridge 350 and of the recorder pen 380 is effected by a variable resistance 378. The element bridges are energized over a tapping switch 394 to provide selective ranges for the recorder. The doublebridge output is passed through a resistance. capacity coupled amplifier 352 to a phaserecognition device 354 comprising triodes 400, 402, the anodes of which are energized from the secondaries 404, 406 of the oscillator 356 output transformer. One or the other of these triodes is energized dependent on whether the output from amplifier 352 is in phase with or opposition to the oscillator voltage introduced by windings 404, 406 and develops a D.C. voltage across the corresponding resistor 408 or 410 proportional to the input to amplifier 352. Phase shift in the amplifier and the input to the phase recognition stage is compensated for by a resistance capacity network 420, 422 and a capacitor 424 ensures that the recognition stage responds only to the in-phase and phase opposition components of the amplifier output. The D.C. voltage across resistor 408 or 410 is impressed on the final D.C. power amplifier stage 360, the output from which is fed to two coaxially pushpull connected solenoid coils 416 of the balancing motor. The power amplifier stage 360 is so arranged that in the absence of any input thereto the two solenoids 416 are equally energized. When the double-bridge circuit is outof-balance the resultant potential impressed on the stage 362 unbalances the currents through the two solenoid coils 416 whereby the magnetic core 418 moves and through a mechanical linkage 374 drives the rotor of the variable condenser 372 in the appropriate direction until balance of the double-bridge is effected when the core 418 stops moving as the currents in the solenoid coils 416 again balance. At the same time the recorder pen 380 is moved to record the strain corresponding to the element bridge in circuit. Automatic sequence switching. The switches 220, 220a.... of Fig. 3 may be operated in any known way, e.g. a three-pole multi-throw switch may be mounted on a shaft which is rotated from one set of contacts to the next by a step-by-step ratchet-wheel actuated by a spring-biassed rocker bar 432, Fig. 6, when the currents in the coils 416 of the solenoid motor are unbalanced and the core 418 thereof is moved to effect balance, a relay winding 426 connected across the solenoid coils 416 is held energized and contacts 427 are closed. Delayed action relay 429 is energized over contacts 427 and contacts 421 closed. When balance is effected by the solenoid motor as above described the currents in the solenoid coils being equal and opposite, the potential across relay 426 drops to a minimum value and contacts 427 open. Relay 429 is de-energized and after a delay contacts 421 open. These contacts 421 remove a short-circuit (over a switch arm 435 in the position shown) from the winding of a motor 431 permanently energized from a source 425 over a current-limiting resistor 425. The motor therefore rotates to drive the chart 382 to its next recording position through mechanism, not shown, and at the same time rotates a notched cam 433 through one revolution. Immediately the cam commences to rotate the switch arm 435 is moved to its lower position where it again energizes relay 429 to close contacts 421. The motor 431 is stopped by short-circuiting it over contacts 421 and the switch arm 435 after the notched cam has completed one revolution to the position